1. Field of the Invention
This invention relates to a mechanism of a pressure-sensitive switch typically used in an automobile refrigerating system to protect the system by inactivating its compressor when the refrigerant pressure in the refrigerating cycle decreases or increases beyond a predetermined value.
2. Description of the Prior Art
The Applicant proposed pressure-sensitive switches disclosed in Japanese Patent Publication Nos. 7-101583(1995) and 7-114094(1995).
A mechanism of a pressure-sensitive switch of this type is explained below with reference to the cross-sectional view shown in FIG. 1. The pressure-sensitive switch is generally composed of a housing 10 which is symmetric about its own axis and defines a fluid passage 11 in its upper portion to permit a fluid to flow in and to transmit its pressure, a pressure-sensitive switching mechanism, and a switch case 20.
The switch case 20 is made of an electrically insulated material, such as polybutylene terephthalate resin reinforced by glass fiber. The switch case 20 defines an interior space containing the pressure-sensitive switching mechanism and opening at its upper end, and a lower opening for electrical connection. The interior space and the lower opening are partitioned by a partition wall 23.
The pressure switching mechanism is made up of a diaphragm 30 made of, for example, a polyimide resin film, first snap disc 31 made of steel, switch receiving member 50 made of polybutylene terephthalate (hereinafter called PBT), operating rod 54, first switch lever 41, second switch lever 42, and a pair of terminals 40.
The outer circumferential portion of the diaphragm 30 of the pressure-sensitive switching mechanism is airtightly secured between the switch case 20 and the housing 10 by a packing 25.
The switch receiving member 50 has a disc-like configuration, having a circular outer wall standing along the upper outer circumferential edge of the disc, a center hole 52, and a center projection 53 made by projecting the circumferential wall of the center hole downwardly from the lower surface of the disc.
The switch receiving member 50 is contained in the interior space of the switch case 20 for sliding movements in the vertical direction.
The inner wall of the interior space of the switch case 20 has a step portion 21 for limiting downward movement of the switch receiving member 50 beyond the step portion 21. Also made below the step portion 21 is a small-diameter step portion 22 supporting the outer circumferential edge of the second snap disc 32.
The first snap disk 31 is supported opposite to the diaphragm 30 by the outer wall 51 of the switch receiving member 50. The operating rod 54 extends through the center hole 52 of the switch receiving member 50 above the second switch lever 42 such that its upper end contacts the lower surface of the first snap disk 31.
The center projection 53 of the switch receiving member 50 engages with the center hole 32a of the second snap disk 30 having an upwardly convexed shape, and the bottom end of the center projection 53 confronts with the center of the upper surface of the first switch lever 41.
The first switch lever 41 has a contact 43 at its distal end, and the second switch lever 42 has a contact 44 at its distal end opposite to the contact 43 of the first switch to form a switch.
A pair of terminals 40 are connected to the first switch lever 41 and the second switch lever 42, respectively, and are introduced to the lower opening of the switch case 20, passing through the partition wall 23.
The pressure-sensitive switch having the above construction operates as explained below.
The pressure of a fluid introduced onto the diaphragm 30 through the fluid passage 11 depresses the diaphragm 30 downwardly, and the switch receiving member 50 supporting the first snap disk 31 is depressed downwardly. When the pressure reaches 250 kPa, for example, the second snap disk 32 deforms from a first aspect, that is upwardly convexed as shown in FIG. 1, to a second aspect that is downwardly convexed. That is, the switch receiving member 50 moves downwardly due to depression by the fluid pressure, and the center projection 53 depresses the first switch lever 41 to close the switch.
The downward movement of the switch receiving member 50 is limited by the step portion 21 on the inner wall of the switch case 20 even when the fluid pressure becomes higher. Therefore, the second snap disk 32 does not receive an excessive force that might make it inoperative due to an unacceptable deformation.
When the fluid pressure decreases to 210 kPa, the second snap disk 32 restores its first aspect from the second aspect, and the switch receiving member 50 moves upwardly, removing its depression from the first switch lever 41. Thus, the switch is opened.
When the fluid pressure further increases to 2.7 MPa after the switch receiving member 50 abuts the step portion 21 and makes the switch ON, the first snap disk 31 deforms from the upwardly convexed first aspect to the downwardly convexed second aspect, and transmits the displacement to the operating rod 54 and then to the second switch lever 42 to press is downwardly. Thus, the switch is opened.
In this manner, the proposed pressure-sensitive switch is turned ON by a pressure within a predetermined range, and is turned OFF by both high pressures and low pressures beyond the predetermined pressure range.
To use the switch in a refrigerating cycle of an automobile refrigerating system, it is typically mounted in an engine room that is held at high temperatures of or beyond 100.degree. C.
As explained above, the conventional pressure-sensitive switch uses PBT resin as the material of the switch receiving member 50. By using the conventional pressure-sensitive switch in circumstances with the temperature held at 100.degree. C. for a long time, the Inventor found the fact that a creep as shown in the broken-lined circle A in FIG. 5 is produced at the foot of the outer wall 51 of the switch receiving member 50 receiving the load of the first snap disk 31 and that the snap disk 31 bites into the surface of the switch receiving member 50.
Thus, the Inventor also found a high possibility that the biting of the outer edge of the snap disk 31 into the foot of the outer wall of the switch receiving member 50 disturbs intended behaviors of the pressure-sensitive switch.